System and method for preventing atrial competition during sensor-driven operation of a dual-chamber pacemaker

ABSTRACT

A system and method for preventing atrial competition during sensor-driven operation of a dual-chamber pacemaker includes means for sensing atrial activity during an atrial refractory period. Atrial competition is avoided by either: (1) generating an atrial competition prevention (ACP) interval upon the detection of any atrial activity during the relative refractory portion of an atrial refractory period, and preventing any atrial stimulation pulses from being generated for the duration of such ACP interval; or (2) shortening the atrial refractory period in the event that the sensor-driven rate of the pacemaker begins to approach a rate that might place atrial stimulation pulses near the end of the unshortened atrial refractory period. Further, the invention includes features that allow monitoring of the intrinsic atrial rate to determine if such is much greater than the sensor-driven rate, and, if so: (1) reducing the maximum tracking rate of the pacemaker, and/or (2) switching the operating mode of the pacemaker from a dual-chamber sensor-driven mode to an single-chamber sensor-driven mode.

This is a divisional of copending application Ser. No. 07/740,554 filedon Aug. 5, 1991.

BACKGROUND OF THE INVENTION

The present invention relates generally to programmable implantablepacemakers, and more particularly, to a system and method for preventingatrial competition in an implantable dual-chamber pacemaker programmedto operate in a sensor-driven mode, i.e., in a mode wherein aphysiological sensor provides an indication of the rate at which thepacemaker should provide pacing pulses on demand.

A brief review of cardiac physiology and pacemaker technology will firstbe presented to help better understand the present invention and theterminology used herein.

The heart is a pump which pumps blood throughout the body. It consistsof four chambers, two atria and two ventricles. In order to efficientlyperform its function as a pump, the atrial muscles and ventricularmuscles must contract in a proper sequence and timed relationship.

In a given cardiac cycle (corresponding to one "beat" of the heart), thetwo atria contract, forcing the blood therein into the ventricles. Ashort time later, the two ventricles contract, forcing the blood thereinto the lungs (right ventricle) or through the body (left ventricle).Meanwhile, blood returning from the body fills up the right atrium andblood returning from the lungs fills up the left atrium, waiting for thenext cycle to begin. A typical healthy adult heart may beat at a rate of60-70 beats per minute (bpm) while at rest, and may increase its rate to140-180 bpm when the adult is engaging in strenuous physical exercise,or undergoing other physiologic stress.

The healthy heart controls its own rhythm naturally from itssinal-atrial (S-A) node, located in the upper portions of the rightatrium. The S-A node generates an electrical impulse at a rate commonlyreferred to as the "sinus" rate. This impulse is delivered to the atrialtissue when the atria are to contract; and, after a suitable delay (onthe order of 120-180 milliseconds), is delivered to the ventriculartissue when the ventricles are to contract.

When the atria contract, a detectable electrical signal referred to as aP-wave is generated. When the ventricles contract, a detectableelectrical signal referred to as an R-wave is generated. The R-wave ismuch larger than the P-wave, principally because the ventricular muscletissue is much more massive than is the atrial muscle tissue. The atrialmuscle tissue need only produce a contraction sufficient to move theblood a very short distance, from the respective atrium to itscorresponding ventricle. The ventricular muscle tissue, on the otherhand, must produce a contraction sufficient to push the blood over along distance, e.g., through the complete circulatory system of theentire body.

Other electrical signals or waves are also detectable within a cardiaccycle, such as a Q-wave (which immediately precedes an R-wave), anS-wave (which immediately follows an R-wave), and a T-wave (whichrepresents the repolarization of the ventricular muscle tissue).

A pacemaker is a medical device that provides electrical stimulationpulses to the appropriate chamber(s) of the heart (atria or ventricles)in the event the heart is unable to beat on its own, i.e., in the eventeither the S-A node fails to generate its own natural stimulation pulsesat an appropriate sinus rate, or in the event such natural stimulationpulses are not delivered to the appropriate cardiac tissue. Most modernpacemakers accomplish this function by operating in a "demand" modewherein stimulation pulses from the pacemaker are provided to the heartonly when it is not beating on its own, as sensed by monitoring theappropriate chamber of the heart for the occurrence of a P-wave or anR-wave. If a P-wave or an R-wave is not sensed within a prescribedperiod of time (which period of time is most often referred to as the"escape interval"), then a stimulation pulse is generated at theconclusion of this prescribed period of time and delivered to theappropriate heart chamber via a pacemaker lead.

Further details associated with cardiac physiology and the operation ofthe heart as controlled or monitored by a pacemaker may be found, e.g.,in U.S. Pat. No. 4,712,555 to Thornander et al.; U.S. Pat. No. 4,788,980to Mann et al.; and/or U.S. Pat. No. 4,944,298 to Sholder. All three ofthese patents are incorporated herein by reference.

Pacemakers are typically both implantable within a patient andprogrammable, allowing their operation to be selectively controlled froma location external to the patient. Modern programmable pacemakers aregenerally of two types: (1) single-chamber pacemakers, and (2)dual-chamber pacemakers. The present invention relates to dual-chamberpacemakers, and more particularly to dual-chamber pacemakers operatingin a rate-responsive mode.

In a single-chamber pacemaker, the pacemaker provides stimulation pulsesto, and/or senses cardiac activity within, a single-chamber of theheart, e.g., either the right ventricle or the right atrium. In adual-chamber pacemaker, the pacemaker provides stimulation pulses to,and/or senses cardiac activity within, two chambers of the heart, e.g.,both the right ventricle and the right atrium. Typically, only the rightatrium and/or the right ventricle is coupled to the pacemaker because ofthe relative ease with which a pacing lead can be transvenously insertedinto either of these chambers. However, the left atrium and leftventricle can also be paced just as effectively providing that suitableelectrical contact is made therewith.

In general, both single and dual-chamber pacemakers are classified bytype according to a three or four letter code. In this code, the firstletter identifies the chamber of the heart that is paced (i.e., thatchamber where a stimulation pulse is delivered), with a "V" indicatingthe ventricle, an "A" indicating the atrium, and a "D" indicating boththe atrium and ventricle. The second letter of the code identifies thechamber wherein cardiac activity is sensed, using the same letters toidentify the atrium or ventricle or both, and wherein a "O" indicates nosensing takes place.

The third letter of the code identifies the action or response that istaken by the pacemaker. In general, three types of action or responsesare recognized: (1) an Inhibiting ("I") response wherein a stimulationpulse is delivered to the designated chamber after a set period of timeunless cardiac activity is sensed during that time, in which case thestimulation pulse is inhibited; (2) a Trigger ("T") response wherein astimulation pulse is delivered to a prescribed chamber of the heart aprescribed period after a sensed event; (3) or a Dual ("D") responsewherein both the Inhibiting mode and Trigger mode are evoked, e.g.,inhibiting in one chamber of the heart and triggering in the other.

The fourth letter, when used, indicates whether the pacemaker isoperating in a sensor-driven mode, i.e., in a mode wherein aphysiological sensor is used to provide an indication of what the rateof the pacemaker should be. Such rate is often referred to as thesensor-indicated rate (SIR). The letter "R" is frequently used for thefourth letter to indicate use of such a sensor-driven mode.

Thus, for example, a DVI pacemaker is a pacer (note that throughout thisapplication, the terms "pacemaker" and "pacer" may be usedinterchangeably) that paces in both chambers of the heart, but onlysenses in the ventricle, and that operates by inhibiting stimulationpulses when prior ventricular activity is sensed. Because it paces intwo chambers, it is considered as a dual-chamber pacemaker. A VVI pacer,on the other hand, is a pacer that paces only in the ventricle andsenses only in the ventricle. A VVIR pacer is a pacer that paces only inthe ventricle at a rate determined by an appropriate physiologicalsensor, and senses only in the ventricle. Because only one chamber isinvolved, a VVI or VVIR pacer is classified as a single-chamberpacemaker.

Most dual-chamber pacemakers can be programmed to operate in any desiredmode, including a single-chamber mode. Hence, e.g., a dual-chamberpacemaker may be programmed to operate in a DDD mode, i.e., a modewherein the pacemaker paces and senses in both the atrium and theventricle. If the dual-chamber pacemaker includes a physiologicalsensor, the dual-chamber pacemaker may be programmed to operate in aDDDR mode, i.e., a mode wherein the pacemaker provides stimulationpulses to both chambers of the heart on demand (i.e., only in theabsence of natural atrial or ventricular activity in the respectivechambers, as determined by sensing in both chambers) at a ratedetermined by the physiological sensor. The present invention addressesa problem that is primarily associated with a dual-chamber pacemakeroperating in a DDDR mode.

One possible effect caused by operating a pacer in a DDD mode is atrialrate based pacing. In an atrial rate based pacemaker, the rate of thepacemaker is set by the heart's S-A node, and the ventricle is paced ata rate following the sensed atrial rate. Because the rate set by the S-Anode represents the rate at which the heart should beat in order to meetthe physiologic demands of the body, at least for a heart having aproperly functioning S-A node, the rate maintained in the ventricle bysuch a pacemaker is truly physiologic. As indicated, a dual-chamberpacemaker, programmed to operate in the DDD mode, provides suchphysiologic pacing. That is, one of the functional states of DDD pacing,particularly applicable to patients having A-V block, is to senseP-waves in the atrium, i.e., to sense the rate set by the S-A node, andpace the ventricle at such sensed rate. Thus, as the physiologic rateincreases, e.g., as the patient exercises and the P-wave rate increases,the pacemaker is able to track such increase and pace the ventricleaccordingly.

Unfortunately, in a conventional DDD pacer, P-waves are tracked only upto a certain limit. If the P-waves occur too rapidly, they begin to fallin what is known as the atrial refractory period (ARP), the relevantportion of which is often referred to as the post ventricular atrialrefractory period (PVARP) because it occurs after ventricular activity,whether such ventricular activity is paced or sensed. During the atrialrefractory period, which is a prescribed time period set by thepacemaker logic circuits, P-waves are not sensed; or, if they aresensed, they are not considered as a P-wave, but are rather consideredas noise. P-waves that occur during the PVARP thus have no effect onpacer timing. The PVARP is intended to provide a sufficient waitingperiod for the heart tissue to settle down or recover following a priordepolarization or contraction. (See, e.g., the previously cited '555patent, and/or the '980 patent, for a more complete description of thetiming intervals, and time periods, measured and/or generated by atypical pacemaker as it performs its function of providing stimulationpulses on demand.)

Thus, if the rate at which P-waves occur increases sufficiently to placea P-wave within the PVARP, such P-wave is not detected by a DDD pacer,and the occurrence of such P-wave has no effect on pacer timing. Thatis, the DDD pacer has no way of knowing that the P-wave occurred, so itwaits until the next P-wave occurs, or until the pacemaker's applicableescape interval times out, whichever occurs first, before initiating theappropriate mechanism for issuing a ventricular stimulation pulse("V-pulse"). Disadvantageously, for a situation where the intrinsicP-waves are gradually increasing, each being followed by a V-pulse, apoint is reached (when the P-wave enters the PVARP) where the intrinsicP-wave is not sensed, resulting in an abrupt decrease in the ventricularpaced rate.

In order to overcome this difficulty --of an abrupt decrease in theventricular rate when tracking P-waves that enter the PVARP--it is knownin the art to utilize a DDDR pacing mode. See Hanich et al.,"Circumvention of Maximum Tracking Limitations with a Rate ModulatedDual-chamber Pacemaker," PACE 12:392-97 (Feb. 1989). Such DDDR pacingmode offers the advantage of providing a sensor-indicated back-up pacingrate after the intrinsic P-waves enter the PVARP. Thus, an abruptdecrease in the ventricular paced rate is avoided because the applicableescape interval in such a rate-responsive pacemaker, e.g., a DDDRpacemaker, is adjusted automatically as a function of the sensor-drivenrate. Hence, as the intrinsic P-wave rate increases due to increasedphysiological demand brought about by, e.g., exercise, the applicableescape interval is shortened by the sensor-driven rate. Thus, eventhough a P-wave may enter the PVARP and not be sensed, the pacemakerwill soon issue an atrial stimulation pulse, followed by a V-pulse, at arate determined by the sensor-driven rate, thereby avoiding abruptchanges in ventricular paced rate.

Disadvantageously, however, once detection of the intrinsic P-wave islost due to its falling within the PVARP, the resulting atrialstimulation pulse ("A-pulse") occurring at the sensor-indicated rate isin competition with the P-wave. Such atrial competition is undesirablebecause it may induce, in many patients, atrial arrhythmias. This isespecially true in those instances where the patient's intrinsic atrialrate has increased due to increased physiological demand, as duringphysical exercise, because during such times the heart is experiencinghigher myocardial oxygen demand and may be experiencing relativeischemia (inadequate flow of blood), both of which conditions mayfurther promote the atrial arrhythmia.

An atrial arrhythmia, if it is short lived, is usually of noconsequence. However, if it persists, it may result in an atrialtachycardia (a very rapid atrial rhythm) or fibrillation, both of whichconditions pose serious health risks to the patient. Hence, what isneeded is a method or technique for preventing atrial competition in apatient having a DDDR pacer, particularly when the DDDR pacer is sensingand tracking intrinsic P-waves that fall into the PVARP of thepacemaker.

Atrial competition also creates other problems. For example, atrialcompetition, by definition, applies an atrial stimulation pulse toatrial tissue as it is repolarizing (i.e., shortly after contraction).This action can significantly desensitize the atrial tissue tosubsequent stimulation pulses, thereby making t difficult to achieve andmaintain "capture" at those times when capture is needed to maintain adesired pacing rate. ("Capture" refers to the response of cardiac tissueto an applied stimulation pulse. When of sufficient energy, astimulation pulse causes cardiac tissue to which it is applied todepolarize and contract; and the cardiac tissue is said to be "captured"by the stimulation pulse. When of insufficient energy, the cardiactissue does not depolarize and contract; and the cardiac tissue does notrespond, i.e., is not captured, by the stimulation pulse.) Thus,applying an A-pulse to the atrium in competition with a P-wave may makesubsequent atrial capture difficult to achieve, as well as introduceatrial arrhythmias. Thus, what is needed is a system and method foroperating a DDDR pacer wherein lack of capture is avoided, and atrialarrhythmias are prevented.

Further, should an atrial arrhythmia persist, there is a need in the artfor a method or technique for quickly terminating such arrhythmia.However, before such terminating technique can be invoked, there is alsoa need for a reliable technique for distinguishing between a true atrialarrhythmia (a potentially dangerous cardiac condition) and a fast atrialrate (which may be a normal and needed response to a high oxygen demandcondition, such as exercise).

Also, it is noted that as a practical matter atrial competition (i.e.,the generating of an A-pulse in competition with an intrinsic P-wave)represents wasted energy of the limited energy resources of theimplanted pacemaker. That is, the atrium, having just naturallydepolarized and contracted, is not capable of depolarizing andcontracting again until such time as the atrial cardiac tissue hasrepolarized. Thus, there is a further need to avoid atrial competitionin order to preserve the limited energy resources of the pacemaker.

Advantageously, the dual-chamber, sensor-driven pacemaker describedherein, including the method of operating such pacemaker, addresses theabove and other needs.

SUMMARY OF THE INVENTION

The present invention provides a system and method for selectivelypreventing atrial competition during operation of the pacemaker in adual-chamber, sensor-driven mode, e.g., the DDDR mode. This isaccomplished by including within the DDDR pacemaker means for sensingatrial activity, e.g., a possible P-wave, during those times of thecardiac cycle, such as during a post ventricular atrial refractoryperiod (PVARP), when such atrial activity would not be sensed by aconventional DDDR pacemaker. Means are also included within thepacemaker, responsive to sensing such atrial activity, for inhibitingany atrial stimulation pulse that would otherwise have been generatedwithout affecting the time at which a ventricular stimulation pulse isgenerated. Hence, competition between the sensed atrial activity and anatrial stimulation pulse is prevented.

Advantageously, the present invention provides two alternate embodimentsfor avoiding atrial competition once atrial activity has been sensed:(1) generating an atrial competition prevention (ACP) interval upon thedetection of any atrial activity during the relative refractory portionof an atrial refractory period, and then preventing any atrialstimulation pulses from being generated during the duration of such ACPinterval; or (2) shortening the atrial refractory period in the eventthe sensor-driven rate of the pacemaker begins to approach a rate thatmight place atrial stimulation pulses near the end of the unshortenedatrial refractory period.

One variation of the first embodiment (generating an ACP interval)inhibits the generation of the atrial stimulation pulse, but otherwisedoes not change the pacemaker timing, i.e., an atrial-ventricular delay(AVD) is initiated at the conclusion of the ventricular-atrial delay(VAD) that is set by the physiological sensor of the pacemaker. Thissensor-controlled AVD is referred to herein as the sensor-indicated rate(SIR) VAD. Another variation of the first embodiment delays thegeneration of the atrial stimulation pulse until the conclusion of theACP interval, thereby in effect extending the SIR VAD until the end ofthe ACP period.

One variation of the second embodiment (shortening the atrial refractoryperiod) involves shortening the atrial refractory period a fixed amount.Another variation involves changing the atrial refractory period, withinprescribed limits, as an inverse function of the sensor-indicated rate(SIR). Thus, if the SIR increases, the atrial refractory period isshortened. If the SIR decreases, the atrial refractory period islengthened.

In accordance with one aspect of the invention, the atrial competitionprevention means may be selectively programmed to be on or off. In theACP interval embodiment, the ACP interval will typically have a value inthe range of 250-350 milliseconds, although any suitable interval couldbe used. In the atrial refractory period shortening embodiment, theatrial refractory period will typically be shortened on the order of50-100 milliseconds.

In accordance with another aspect of the invention, there is provided ameans for distinguishing potentially dangerous atrial arrhythmias fromnormal fast atrial rates for a pacemaker operating in the DDDR mode, andmeans for terminating such atrial arrhythmias. The distinguishing meansincludes means for sensing the occurrence of a high intrinsic atrialrate and comparing such rate to the sensor-driven rate. A high intrinsicatrial rate in the absence of a corresponding high sensor-driven rate ispresumed to be an atrial tachycardia condition--a potentially dangerouscondition that needs to be terminated as quickly as possible.(Alternatively, such determination may indicate a malfunction of thepacemaker circuits and/or sensor responsible for generating thesensor-driven rate.) The terminating means, which operates only inresponse to detecting a high intrinsic atrial rate in the absence of acorresponding high sensor-driven rate includes means for automatically:(1) reducing the maximum tracking rate of the pacemaker; and/or (2)switching the operating mode of the pacemaker from a dual-chambersensor-driven mode to a single-chamber sensor-driven mode, such as theVVIR mode. Advantageously, reducing the maximum tracking rate limits therate at which the pacemaker can provide stimulation pulses. Switchingthe operating mode to a VVIR mode eliminates the possibility of atrialcompetition by eliminating any possibility of generating atrialstimulation pulses. Either of these responses, in turn, reduces theeffects of the fast atrial rate condition.

Moreover, in the event the circuits/sensor responsible for generatingthe sensor-driven rate have somehow malfunctioned, neither of theresponses to a detected disparity between the intrinsic atrial rate andthe sensor-driven rate comprise responses that could worsen thesituation until such time as the circuits/sensor can be replaced orrepaired.

It is thus a feature of the invention to provide a dual-chamberpacemaker operating in a sensor-driven mode, and a method of operatingsuch a pacemaker, that avoids atrial arrhythmias.

It is another feature of the invention to provide such a dual-chamber,sensor-driven pacemaker and method of operation that ensures that whenan atrial stimulation pulse is provided, it captures the heart.

It is still another feature of the invention to provide a system andmethod for operating a dual-chamber, sensor-driven pacemaker thatminimizes the likelihood of atrial competition between naturallyoccurring P-waves and atrial stimulation pulses which could induce anatrial arrhythmia.

It is a further feature of the invention, in accordance with oneembodiment thereof, to provide such a system and method for operating adual-chamber, sensor-driven pacemaker that detects atrial activityduring the pacemaker's atrial refractory period and prevents any atrialstimulation pulses from being generated within a prescribed timeinterval thereafter, thereby minimizing atrial competition by assuringthat there is always at least the prescribed time interval betweensensed atrial activity and an atrial stimulation pulse.

It is an additional feature of the invention, in accordance with anotherembodiment thereof, to provide such a system and method for operating adual-chamber, sensor-driven pacemaker that automatically shortens thepacemaker's atrial refractory period (ARP) whenever the sensor-drivenrate approaches a rate that might place atrial stimulation pulses nearthe end of the time in the cardiac cycle when the original (unshortened)ARP terminates, thereby minimizing atrial competition by increasing thetime period (after the ARP) during which atrial activity can be sensed,which atrial activity (if sensed after the ARP) inhibits the generationof an atrial stimulation pulse.

It is yet a further feature of the invention to provide a dual-chamber,sensor-driven pacemaker that includes means for detecting an atrialarrhythmia whenever there is a large disparity between a sensed atrialrate and the sensor-driven rate, and that further includes means forautomatically dissociating the ventricular paced rate from the detectedatrial rate, thereby perhaps reducing patient symptoms, and terminatingsuch atrial arrhythmia once detected. In accordance with this feature,one embodiment of the invention causes the maximum tracking rate of thepacemaker to be reduced in response to a sensed atrial arrhythmia.Another embodiment causes the pacing mode of the pacemaker toautomatically switch to a single-chamber sensor-driven mode, such asVVIR mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is block diagram o a dual-chamber, programmable, rate-responsivepacemaker;

FIG. 2 is a block diagram of one possible embodiment of the controllogic of the pacemaker of FIG. 1;

FIG. 3 is a timing diagram illustrating a series of waveforms andpacemaker timing intervals (A)-(F) that depict the problem of atrialcompetition, and further show how variations of one embodiment of thepresent invention addresses this problem by generating an atrialcompetition prevention (ACP) interval; and

FIGS. 4, 5, 6 and 7 are flowchart diagrams illustrating the operation ofthe pacemaker of FIG. 1 in accordance with representative embodiments ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

Before describing the present invention, it will be useful to describethe general framework of a programmable dual-chamber rate-responsivepacemaker, as such framework provides the basis within which the presentinvention is carried out. Accordingly, reference will first be made toFIGS. 1 and 2, where there is shown, respectively, a block diagram of adual-chamber programmable pacemaker, and a block diagram of the controllogic used within such a pacemaker. Once such framework has beendescribed, the present invention will be described with reference to thetiming diagrams of FIG. 3 and the flowchart diagrams of FIGS. 4, 5 and6.

Referring first then to FIG. I, a simplified block diagram of adual-chamber pacemaker 10 is illustrated. The pacemaker 10 is coupled toa heart 12 by way of leads 14 and 16, the lead 14 having an electrode 15which is in contact with one of the atria of the heart, and the lead 16having an electrode 17 which is in contact with one of the ventricles ofthe heart. The leads 14 and 16 carry stimulating pulses to theelectrodes 15 and 17, respectively, from an atrial pulse generator(A-PG) 18 and a ventricular pulse generator (V-PG) 20, respectively.These stimulating pulses may be referred to herein as the "A-pulse" orthe "V-pulse".

Further, electrical signals from the atria are carried from theelectrode 15, through the lead 14, to the input terminal of an atrialsense amplifier (P-AMP) 22. Electrical signals from the ventricles arecarried from the electrode 17, through the lead 16, to the inputterminal of a ventricular sense amplifier (R-AMP) 24.

Controlling the dual-chamber pacer 10 is a control system 26. Thecontrol system 26 receives the output signals from the atrial senseamplifier 22 over a signal line 28. Similarly, the control system 26receives the output signals from the ventricular sense amplifier 24 overa signal line 30. These output signals are generated each time that aP-wave or an R-wave is sensed within the heart 12.

The control system 26 also generates trigger signals which are sent tothe atrial pulse generator 18 and the ventricular pulse generator 20over two signal lines 32 and 34, respectively. These trigger signals aregenerated each time that a stimulation pulse is to be generated by therespective pulse generator 18 or 20.

During the time that either an A-pulse or V-pulse is being delivered tothe heart, the corresponding sense amplifier, P-AMP 22 or R-AMP 24, istypically disabled by way of a blanking signal presented to these senseamplifiers from the control system over signal lines 36 and 38,respectively. This blanking action prevents the sense amplifiers 22 and24 from becoming saturated from the relatively large stimulation pulseswhich are present at their input terminals during this time. Thisblanking action also helps prevent residual electrical signals presentin the muscle tissue as a result of the pacer stimulation from beinginterpreted as P-waves or R-waves.

Still referring to FIG. 1, the pacer 10 also includes a memory circuit40 which is coupled to the control system 26 by a suitable data/addressbus 42. This memory circuit 40 allows certain control parameters, usedby the control system 26 in controlling the operation of the pacemaker,to be programmably stored and modified, as required, in order tocustomize the operation of the pacer 10 to suit the needs of aparticular patient. Further, data sensed during the operation of thepacer 10 may be stored in the memory 40 for later retrieval andanalysis.

A telemetry circuit 44 is further included in the pacer 10. Thistelemetry circuit 44 is connected to the control system 26 by way of asuitable command/data bus 46. In turn, the telemetry circuit 44, whichis included within the implantable pacer 10, may be selectively coupledto an external programming device 48 by means of an appropriatecommunication link 50, which communication link 50 may be any suitableelectromagnetic link, such as an RF (radio frequency) channel.

Advantageously, through the external programmer 48 and the communicationlink 50, desired commands may be sent to the control system 26.Similarly, through this communication link 50 and the programmer 48,data (either held within the control system 26, as in a data latch, orstored within the memory 40,) may be remotely received from the pacer10. In this manner, noninvasive communications may be established withthe implanted pacer 10 from a remote, non-implanted, location.

The pacer 10 in FIG. 1 is referred to as a dual-chamber pacemakerbecause it interfaces with both the atria and the ventricles of theheart. Those portions of the pacer 10 which interface with the atriae.g., the lead 14, the atrial sense amplifier 22, the atrial pulsegenerator 18, and corresponding portions of the control system 26, arecommonly referred to as the atrial channel. Similarly, those portions ofthe pacer 10 which interface with the ventricles, e.g., the lead 16, theventricular sense amplifier 24, the ventricular pulse generator 20, andcorresponding portions of the control system 26, are commonly referredto as the ventricular channel.

In accordance with the present invention, the pacemaker 10 furtherincludes a physiological sensor 52 which is connected to the controlsystem 26 of the pacer over a suitable connection line 54. While thissensor 52 is illustrated in FIG. 1 as being included within the pacer10, it is to be understood that the sensor 52 may also be external tothe pacer 10, yet still be implanted within or carried by the patient.

A common type of sensor 52 is an activity sensor, such as apiezoelectric crystal, which senses physical motion (activity) of thepatient. Such sensor is typically mounted to the can or case of thepacemaker. Other types of physiologic sensors are also known, such assensors which sense the oxygen content of blood, respiration rate, pH ofblood, repolarization time of the heart, and the like. The type ofsensor used is not critical to the present invention. Any sensor whichis capable of sensing some parameter which is relatable to thephysiological rate at which the heart should be beating may be used.Physiological sensors of the type described are commonly used with"rate-responsive" pacemakers in order to adjust the rate (escapeinterval) of the pacer in a manner which tracks the physiological needsof the patient. Thus, stimulation pulses are generated only on demand(in the absence of naturally occurring cardiac activity) at a ratedetermined by the physiological sensor.

Referring next to FIG. 2, a block diagram of one embodiment of thecontrol system 26 of the pacer 10 is illustrated. It is noted that otherembodiments of a control system 26 may also be utilized, such as amicroprocessor-based control system. A representativemicroprocessor-based system is described, for example, in U.S. Pat. No.4,940,052, entitled "Microprocessor Controlled Rate-Responsive PacemakerHaving Automatic Rate Response Threshold Adjustment," assigned to thesame assignee as is the present application. The '052 patent isincorporated herein by reference.

The control system shown in FIG. 2 is based on a state machine wherein aset of state registers 60 define the particular state of the pacer 10 atany instant in time. In general, and as an overview of state machineoperation, each state, by design, causes a certain activity or functionto be carried out. Several states are executed in a sequence during agiven cardiac cycle. The sequence of states which is executed in aparticular cardiac cycle is determined by the particular events whichoccur, such as the sensing of a P-wave or an R-wave, as well as thecurrent state, as certain states can only be entered from certain otherstates.

Only one state may exist at any instant of time, although severaldifferent state machines (or control systems) may operate in parallel tocontrol diverse functions. For example, the telemetry circuit 44(FIG. 1) preferably utilizes its own state machine, such as is describedin the above-cited '052 patent. This telemetry circuit state machineoperates essentially independently of the control system state machineshown in FIG. 2.

At the heart of the control system 26 is the state logic 62. It is thestate logic which controls the "state" of the state registers 60 andhence the function or operation which will next be carried out by thesystem. The state logic 62 receives as inputs the current state of thestate registers 60, made available over a state bus 64 (which state bus64 directs the state of the system to several sections of the controlsystem), as well as other signals indicating the current status of thesystem or events which have occurred.

The output signals from the P-AMP 22 (FIG. 1) and the R-AMP 24 (FIG. aredirected to an input decode logic circuit 66. The input decode logiccircuit 66 generates appropriate logic signals "IPW" (Inhibiting P-Wave)and "IRW" (Inhibiting R-Wave) which are selected by a multiplexer 68 andsent to rate-determining logic 70. These signals are also sent to thestate logic 62. The function of the rate-determining logic 70 is todetermine the rate at which either the IPW or IRW signals are occurring.

A signal representative of this rate is sent, as an output signal fromthe rate determining logic 70, to the state logic 62 over a signal line72. The rate-determining logic 70 further receives a sensor rate signalfrom the sensor 52 (FIG. 1), and (depending upon the particular state ofthe system, as defined by the state registers 60, and as made availableto the rate-determining logic 70 over the state bus 64) sends a ratesignal to the state logic 62 over signal line 72 indicative of thissensor rate.

Still referring to FIG. 2, a memory control circuit 74 provides theneeded interface between the circuits of the control system 26 and thememory 40 (FIG. 1). This memory control circuit 74 may be anyconventional memory access circuit which sends or receives data to orfrom memory at a specified address. Data retrieved from the memory 40may be sent to either the state logic 62 over signal line(s) 75 or to aprogrammable timer 76 over a signal line(s) 77. Data sent to the memory40 may be either the current state of the system (obtained off of thestate bus 64), or other selected signals from the state logic 62 (asmade available over signal line(s) 73).

The function of the programmable timer 76 is to define a prescribed timeinterval, the length of which is set by the signal(s) received from thememory control 74 over the signal line(s) 77, and the starting point ofwhich begins coincident with the start of the current state, as obtainedfrom the state bus 64. The timer 76 further generates a time-out signalwhen this prescribed time interval has elapsed.

During this prescribed time interval, the timing function may be resetby a reset signal, typically obtained from the input decode logic 66,although some states (as obtained from the state bus 64) may alsoeffectuate an immediate reset of the timer 76. The time-out signal issent to a time-out decode logic 78. It is the function of the time-outdecode logic 78 to generate the appropriate trigger signals which aresent to the A-pulse generator 18 or the V-pulse generator 20 (FIG. I).Further, an appropriate logic signal(s) is sent to the state logic 62 bythe time-out decode logic 78 over the signal line(s) 80 in order tonotify the state logic 62 that the respective trigger signals have beengenerated.

An oscillator 82, preferably a crystal-controlled oscillator, generatesa basic clock signal C0 which controls the operation of the systemlogic. This clock signal C0 is sent to clock logic circuits 84, whereother appropriate clock signals, such as clock signals C1, C2 and C3,are generated, all derived from the basic clock signal C0. These clocksignals are distributed throughout the control system 26 in order toappropriately synchronize the various events and state changes whichoccur within the pacemaker.

The rate of the basic clock signal C0 is not critical to the presentinvention. In general, a rate of 25-40 Khz for the basic clock rate C0is adequate. This rate provides a basic time increment of 25-40microseconds each clock cycle, and this is more than enough time toeffectively control the pacemaker operation. If desired, a faster basicclock rate may be used, particularly by the memory control 74, to speedup the data transfer between the control system 26 and the memory 40,although for most pacemaker operations, a fast data transfer rate is notessential.

In operation, the control system of FIG. 2 starts at an initial state,wherein the state registers 60 assume a prescribed value which definesthe initial state. For example, assuming four flip-flops are used forthe state registers 60, an initial state might be "1000" (hexadecimal"8") wherein the first flip-flop assumes a "1" state, and the remainingthree flip-flops each assume a "0" state. This state may be defined as aV-A Delay (VAD) state wherein a prescribed VA interval is initiated.This interval may be considered as the "escape interval" mentionedpreviously.

As soon as the memory control 74 detects that the VAD state has beeninitiated, as evidenced by the "1000" appearing on the state bus 64, itretrieves from the memory 40 an appropriate data word, previouslyprogrammed into the memory 40 from the external programmer 48, whichdefines the desired length of the V-A delay. This data word is sent tothe programmable timer and sets the length of the time period which isto be measured during the VAD state.

The timer 76 is essentially just a counter which counts down (or countsup), using a specified clock signal, to the value specified in the dataword. When the counting has been completed, and assuming that thecounter has not been reset by the occurrence of a P-wave or an R-wave,the counter or timer 76 is said to have "timed-out," and an appropriatetime-out signal is generated which is sent to the time-out decode logic78.

The decode logic 78, in turn, recognizes that the current state of thesystem is the VAD state (as determined by monitoring the state bus 64),and therefore that the VA interval (escape interval) has timed outwithout any cardiac activity having been sensed, generates an A-pulsetrigger signal, sent to the A-pulse generator 18, so that the atrium canbe stimulated. At the same time, an appropriate logic signal(s) is sentto the state logic 62 over the signal line(s) 80 to alert the statelogic to the fact that the timer 76 has timed out.

The state logic 62, in response to receiving the signal(s) from thetime-out decode logic 78, and also in response to the current VAD state,triggers the next state of the prescribed sequence. For DDD operation,this state is typically a blanking state, or BLANK state, during whichthe P and R sense amplifiers, 22 and 24, are disabled. Accordingly, thestate logic generates appropriate signal(s) on signal lines 36 and 38 toblank the atrial sense amplifier 22 and the ventricular sense amplifier24, respectively, and also causes the state registers 60 to change to aBLANK state, which state could be defined, for example, by theflip-flops of the state registers 62 assuming a "0001" (hex "1")condition.

This BLANK state, detected on the state bus 64, causes the memorycontrol circuitry 74 to retrieve an appropriate data word from thememory 40 which defines the length of the blanking interval, which dataword is loaded into the programmable timer 76. As soon as the timer 76times out, indicating that the prescribed blanking interval has elapsed,a time-out signal is generated which is sent to the time-out decodelogic 78. Upon receipt of this time-out signal, and in response to thecurrent state being a BLANK state, the time-out decode logic 78 sends anappropriate logic signal to the state logic 62. The state logic 62responds by steering the state registers 60 to assume the next state inthe prescribed sequence, which may be, for example, an A V Delay (AVD)state. At the beginning of the AVD state, another value is loaded intothe programmable timer 76 which defines the length of the AV interval.If the timer 76 times out without being reset, indicating that noP-waves or R-waves have been sensed, the decode logic 78 generates aV-pulse trigger signal, and notifies the state logic 62 of this event.The state logic 62, in turn, causes the next appropriate state to beentered, which state may be another blanking state, or BLANK state,similar to the one described above, but having perhaps a differentduration. At the conclusion or timing out of this second BLANK state,the next state in the prescribed sequence is initiated, which state maybe a refractory (REF) state.

In the manner described above, the control system 26 assumes one stateafter another, thereby controlling the operation of the pacemaker Ingeneral, a state is changed when the timer 76 times out, or when aprescribed event occurs. For example, if during the VAD state an IPWsignal is received (indicating that a P-wave has been sensed), the inputdecode logic 66 generates a reset signal to reset the timer 76, and thestate logic 62 responds by immediately (typically within the next fewclock cycles) changing the state to the next appropriate state, forexample, an AVD state.

Further, if during the AVD state an IRW signal is received (indicatingthat an R-wave has been sensed), the input decode logic 66 generatesanother reset signal to reset the timer 76, and the state logic respondsby immediately changing the state to the next appropriate state, forexample, a refractory (REF) state. It is noted that the state of thecontrol system 26 could also be changed by receipt of an appropriatecommand from the telemetry system.

The control system 26 of FIG. 2 may be realized using dedicated hardwarecircuits, or by using a combination of hardware and software (orfirmware) circuits. The appropriate sequence of states for a given modeof operation, such as DDD or VVI, for example, may be defined byappropriate control of the memory control 74 and the state logic 62.These circuit elements, in turn, are most easily controlled through anappropriate software or firmware program which is placed or programmedinto the pacemaker memory circuits. The manner of accomplishing suchprogramming is well known in the art. A detailed description of thevarious circuits of the control system 26 of FIG. 2 will not bepresented herein because all such circuits may be conventional, or maybe patterned after known circuits available in the art. Reference ismade, for example, to the above-referenced '555 patent, to Thornander etal., wherein a state-machine type of operation for a pacemaker isdescribed; and to the '980 patent, to Mann et al., wherein the varioustiming intervals used within the pacemaker and their interrelationshipare more thoroughly described.

The operating states of a typical dual-chamber programmable pacemakermay have up to eighteen states associated with its control system. Thesestates are described fully in the above-referenced patents. A summary ofthese states is presented below in Table 1.

                  TABLE 1                                                         ______________________________________                                        States of the Pacemaker Control System                                        State  Symbol      Description                                                ______________________________________                                        0      APW         A-Pulse (A-Pulse triggered)                                1      BLANK       V-Sense Input Inhibit (Blank)                              2      AREF        A Refractory                                               3      SIPW        Sensed Inhibiting P-wave (P-                                                  wave sensed)                                               4      AVD         A-V Delay                                                  5      CROSS       Crosstalk Sense                                            6      VPW         V-Pulse (V-Pulse triggered)                                7      SIRW        Sensed Inhibiting R-wave (R-                                                  wave sensed)                                               8      VAD         V-A Delay                                                  9      SHORT1      Shorten A-V Delay a first                                                     prescribed amount if IPW                                                      during SHORT1 with Physiologic                                                A-V Delay On                                               A      MTR         Max Track Rate                                             B      SHORT2      Shorten A-V Delay a second                                                    prescribed amount if IPW                                                      during SHORT2 with Physiologic                                                A-V Delay On                                               C      RRT         Lengthen VA interval if at low                                                battery                                                    D      RNOISE      R Noise sensed during VREF or                                                 RNOISE                                                     E      LIPW        Latched IPW -- P-wave sensed                                                  in MTR                                                     F      PNOISE      P Noise sensed during AREF or                                                 PNOISE                                                     (none) VREF        V Refractory, independent 1-                                                  bit state machine synchronized                                                to pulse generator when AREF                                                  starts                                                     (none) ABSREF      Absolute Refractory for a                                                     prescribed period, starts when                                                AREF starts                                                ______________________________________                                    

In addition to the states identified above in Table 1, a dual-chamberpacemaker made in accordance with the present invention preferablyincorporates at least two additional states: (1) an ACP (AtrialCompetition Prevention) state, and an ARV (Atrial Rate Verify) state, aswill become evident from the description that follows.

With the foundation of a dual-chamber, rate-responsive, programmablepacemaker now established, the present invention will be more fullydescribed. Broadly speaking, one embodiment of the invention may becharacterized as a system for preventing atrial competition in arate-responsive, dual-chamber pacemaker configured to operate in a DDDRmode of operation. Such system includes: (a) means for defining aphysiological pacing rate; (b) control means for generating timingsignals indicative of when an atrial and/or ventricular stimulationpulse should be generated by the pacemaker in order to maintain thephysiological pacing rate; (c) sensing means coupled to the controlmeans for sensing atrial and ventricular activity, such as P-waves,indicating natural atrial activity, and R-waves, indicating naturalventricular activity, the control means generating the timing signalsneeded to generate atrial and/or ventricular stimulation pulses ondemand as needed in the absence of intrinsic P-waves and/or R-waves; and(d) stimulation pulse generating means coupled to the control means forgenerating the atrial and/or ventricular stimulation pulses in responseto the timing signals. The control means in such embodiment may becharacterized as comprising: (i) PVARP generating means for generating apost ventricular atrial refractory period (PVARP) subsequent to thegeneration of each ventricular stimulation pulse or the sensing of anR-wave, the PVARP defining a time interval during which sensed atrialactivity is not considered as a valid P-wave, and (ii) atrial pulseprevention means generates an atrial stimulation pulse from beinggenerated that is in competition with atrial activity sensed during thePVARP. Advantageously, the atrial pulse prevention of such embodimentoccurs without changing the timing signals that control when aventricular stimulation pulse is generated in order to maintain thephysiological pacing rate.

In one particular embodiment of this invention, the atrial pulseprevention means generates an atrial competition prevention (ACP)interval in response to atrial activity sensed during the PVARP. SuchACP interval has a prescribed duration. The generation of any atrialstimulation pulses during the ACP interval is inhibited or delayed untilthe end of the ACP interval. Hence, an atrial pacing pulse is notgenerated in competition with sensed atrial activity that occurs duringthe PVARP for at least the duration of the ACP interval.

The operation of such atrial pulse prevention means is best illustratedwith reference to FIG. 3. FIG. 3 shows a timing diagram illustrating aseries of waveforms and pacemaker timing intervals, labeled (A)-(E). Ingeneral, these series of waveforms depict the problem of atrialcompetition, and further show how the generation of an atrialcompetition prevention (ACP) interval minimizes such problem.

It is noted that in the timing and waveform diagrams of FIG. 3, severalevents are shown as a function of time. In each of the five differentsequences of events shown in FIG. 3, cardiac events are represented byan electrocardiograph (ECG) schematic that includes P-waves, A-pulses,or V-pulses. The little "bump" by the letter "P" represents a P-wave.The vertical line by the letter "V" represents a V-pulse. The largewaveform following a V-pulse represents the QRS-T waves that accompany astimulated ventricular depolarization. The vertical line by the letter"A" represents an A-pulse. Timing intervals, time periods, or timedelays, generated by the pacemaker control system, i.e., the statelogic, are represented as boxes or rectangles above the ECG waveform.For simplicity, only time intervals relevant to the present inventionare shown. A given timing interval has a relative duration within thecardiac cycle as shown in the figure. That is, a given timing intervalbegins at that instant of time coincident with its left edge, andterminates at that instant of time coincident with its right edge, withtime being the horizontal axis and increasing to the right.

In the upper waveform diagram of FIG. 3, labeled (A), hereafter referredto as FIG. 3(A), conventional atrial rate based pacing (P-wave tracking)is shown. During such pacing, the occurrence of a P-wave 90 causes theA-V delay (AVD) 92 to begin, i.e., causes the AVD state to be entered.At the conclusion of the A-V delay 92, a V-pulse 94 is issued. Thegeneration of the V-pulse 94, in turn, causes the ventricles tocontract, represented by the QRS wave 96 and T-wave 97. Also, thegeneration of the V-pulse causes the post ventricular atrial refractoryperiod (PVARP) 98 to be generated. During the PVARP 98, no atrialactivity can be sensed; or, if it is sensed, it is not treated as aP-wave. For the situation shown in FIG. 3(A), no P-waves occur duringthe PVARP 98. However, after PVARP has terminated, a P-wave 100 occurs.This P-wave 100 is sensed by the pacemaker circuits as a P-wave, andcauses another A-V delay 102 to be initiated. At the conclusion of theA-V delay, a V-pulse 104 is issued, causing the ventricles to againdepolarize. This process continues, with each V-pulse being issued oneA-V delay after the sensing of a P-wave. Thus, the ventricle is paced ata rate that tracks the sensed P-waves.

In FIG. 3(B), a condition is shown where it is assumed that intrinsicP-waves occur at a relatively rapid rate, e.g., as might occur duringphysical exercise. A first P-wave 105 is sensed and triggers the A-Vdelay 107, after which a V-pulse 108 is issued. The V-pulse 108 causesthe ventricles to contract, and also initiates the PVARP 109. A secondP-wave 110 occurs during the PVARP 109. Thus, in a conventional DDDpacemaker, the P-wave 110 is not recognized by the pacemaker logiccircuits as a P-wave, but is rather considered as noise. Hence, thepacemaker logic circuits are not aware that P-wave 110 has occurred, andthey remain armed in an appropriate waiting state, waiting for the nextP-wave to occur, or for the applicable escape interval to time out,whichever occurs first. Because, the intrinsic P-wave rate is relativelyfast, a third P-wave 111 occurs after the expiration of the PVARP 109,and before the appropriate escape interval times out. Hence, the P-wave111 is sensed, causing the A-V delay 112 to be initiated. After the A-Vdelay 112 times out, a V-pulse 113 is generated, causing the desiredventricular contraction. This process continues with every other P-wavebeing sensed, and tracked. This condition (of sensing every otherP-wave) is referred to as 2:1 block. It is not desirable to remain in a2:1 block condition for any sustained period of time because the heartis only being paced at a rate that is one half as fast as it should bein order to pump the needed blood supply through the body to meet thephysiological demands manifest by the rapid intrinsic P-wave rate.

In order to alleviate the problem of 2:1 block, as well as otherproblems, the DDDR pacing mode may be used. Operation in a DDDR pacingmode is depicted in FIG. 3(C). In such a pacing mode, it is againassumed that there is a relatively fast intrinsic P-wave rate. A firstP-wave 115 is sensed, causing the A-V delay 116 to be generated. At theconclusion of the A-V delay 116, a V-pulse 117 is issued, and a PVARP118 is initiated, as described previously. However, because a DDDRpacing mode is being used, the V-pulse 117 also causes asensor-indicated rate (SIR) V-A delay (VAD) 119 to be operative. ThisSIR VAD 119 essentially represents the escape interval of the pacemakeras determined from the physiologic sensor. Because there is a relativelyfast intrinsic P-wave rate, which fast P-wave rate evidences a highphysiologic demand, the SIR VAD 119 will also represent a highphysiologic demand, assuming the physiologic sensor is functioningproperly. That is, the SIR VAD will not be very long. Thus, as shown inFIG. 3(C), even though the next P-wave 120 occurs during PVARP 118, andis thus not sensed (as was the case in FIG. 3(B) above), the SIR VAD 119terminates soon after the termination of PVARP, causing an A-pulse 121to be generated. The generation of the A-pulse 121 triggers an A-V delay122, after which a V-pulse 123 is generated, thereby allowing the heartto be paced at a relatively rapid rate commensurate with the sensedphysiological need for a rapid heart rhythm.

Unfortunately, the condition shown in FIG. 3(C) creates atrialcompetition between the P-wave 120 and the A-pulse 121. Atrialcompetition is not desirable for the reasons previously explained.Advantageously, the present invention prevents such atrial competitionusing the techniques and/or methods described below.

In accordance with an atrial competition prevention (ACP) embodiment ofthe invention, atrial activity is sensed during the PVARP (at leastduring the relative refractory portion of the PVARP). The sensing ofatrial activity during PVARP causes an atrial competition prevention(ACP) time interval to be generated, i.e., causes an atrial competitionprevention state to be entered by the state logic that lasts for aprescribed period of time. The prescribed duration of the ACP interval,or state, is fixed, e.g., on the order of 250-350 milliseconds, asselected by a physician at the time of programming the pacemaker. Thepacemaker logic is configured so that during the ACP interval, no atrialstimulation pulse, A-pulse, can be generated, even if one is called forby the SIR VAD (sensor-driven escape interval). If the SIR VAD times outduring the ACP interval, the A-pulse is inhibited, but the A-V delay isinitiated as though an A-pulse had been issued. Or, alternatively, theSIR VAD is simply extended until the end of the ACP window. If the SIRVAD times out after the expiration of the ACP interval, an A-pulse isissued in normal fashion. This assures that there will always be a timeinterval equal to at least the duration of the ACP interval between anyatrial activity sensed during the PVARP and an A-pulse, therebyminimizing the likelihood of atrial arrhythmia induction due tocompetition.

Operation of the ACP embodiment of the invention is shown in FIGS. 3(D),3(E), and 3(F). In FIG. 3(D), a P-wave 130 occurs during the PVARP 128.Thus, an ACP interval 129 is initiated. Before the timing out of thisACP interval 129, the SIR VAD times out. Hence, no A-pulse is generated.However, an A-V delay 134 is initiated at the conclusion of the SIR VAD.When the A-V delay 134 times out, a V-pulse 135 is generated, therebycontinuing to pace the heart at the sensor-driven rate, despite the factthat no A-pulse was generated.

In FIG. 3(E), a P-wave 136 likewise occurs during the PVARP 137. Hence,an ACP interval 131 is initiated. In accordance with the variation ofthe invention illustrated in FIG. 3(E), the associated SIR VAD 133 isextended to the end of the ACP interval 131. An A-pulse 139 is generatedat the end of the SIR VAD 133, i.e., at the end of the ACP interval 131.As is normal, an A-V delay 143 is generated when the extended SIR VAD133 times out. When the A-V delay 146 times out, a V-pulse 147 isgenerated, thereby pacing the heart at a rate that is slightly modifiedfrom the sensor-driven rate.

In FIG. 3(E), a p-Wave 140 likewise occurs during a PVARP 138, therebyinitiating an ACP interval 141. When the ACP interval 141 times out, theSIR VAD 142 has not timed out. Hence, at the timing out of the SIR VAD142, an A-pulse 143 is generated, causing the A-V delay 144 to start. Atthe conclusion of the A-V delay 144, a V-pulse 145 is generated. Thus,the heart is paced at the sensor-driven rate.

It is noted that in all of the sequences shown in FIG. 3, it is assumedthat there is no natural ventricular activity. However, it is to beunderstood that should an R-wave be sensed during the A-V delay, noV-pulse will be generated. That is, the pacemaker delivers stimulationpulses to the heart, whether A-pulses or V-pulses, only on demand asindicated by the SIR VAD or AVD. However, even when an A-pulse isdemanded at the conclusion of the SIR VAD, such will not be providedunless the ACP interval has also timed out.

In another particular embodiment of the invention, referred to as anatrial rate verify (ARV) embodiment, two additional variations of theinvention are provided. In a first ARV variation, the atrial pulseprevention means includes: (a) rate determining means for determining anintrinsic atrial rate; (b) first comparison means for determining if thesensed intrinsic atrial rate is approaching a reference rate; and (c)means responsive to the first comparison means for shortening theduration of the PVARP. Shortening PVARP in this manner allows atrialactivity to be sensed and recognized by the pacemaker logic circuitry asa valid P-wave, thereby inhibiting the generation of any atrialstimulation pulse in accordance with conventional demand pacemakeroperation. For example, with reference to FIG. 3(C), if the PVARP 118 isshortened by an amount represented by the portion 150, then P-Wave 120is sensed, and the A-pulse 121 would be inhibited in conventional demandpacemaker operation.

In a second ARV variation, the atrial pulse prevention means includes:(a) rate determining means for determining an intrinsic atrial rate; (b)first comparison means for determining if the sensed intrinsic atrialrate is equal to or greater than a reference rate; and (c) meansresponsive to the first comparison means for changing the duration ofthe PVARP as an inverse function of the sensor-indicated rate, or SIR.That is, the duration of the PVARP is inversely tied to the SIR. Thus,if the SIR increases, the PVARP is shortened, and the benefits of ashortened PVARP are obtained as in the first ARV variation describedabove. However, as the SIR decreases, returning to its initial value,the PVARP lengthens, also returning back to its initial value. Themanner in which the PVARP is controlled by the SIR is controlled by anappropriate algorithm, which algorithm could, of course, be programmedon or off. When on, there is a prescribed relationship between the SIRand PVARP. For example, the algorithm may be such that for every 10 ppm(pulses per minute) increase in the SIR, the PVARP is reduced by 10-20milliseconds. Of course, these values are only exemplary, and anysuitable inverse relationship could be set between the SIR and thePVARP.

Thus, in operation, the ARV embodiments of the invention monitor therate of the intrinsic P-waves. If such rate is increasing sufficientlyto likely place a P-wave within the normal PVARP, then an ARV state isentered by the state logic that either causes the PVARP to be shortenedby a prescribed amount, such as 50 to 100 milliseconds, or that shortensthe PVARP by an amount dictated by the increase in the SIR. Bypreventing the intrinsic P-wave from falling within the PVARP (byshortening the normal PVARP), the intrinsic P-wave is sensed as aP-wave, and such sensing inhibits any A-pulse generation in conventionaldemand pacemaker operation. If the monitored intrinsic P-wave ratesubsequently slows down so that there is no danger of the P-wavesfalling within the normal PVARP, then the shortened PVARP is extendedback to the normal PVARP, either in a single step or gradually as theSIR rate returns to normal. That is, as the P-wave rate subsequentlyslows down, the ARV state is ended, and the pacemaker returns to itsnormal DDDR operation.

Referring next to FIGS. 4, 5 and 6, flowchart diagrams illustrating amethod of operating the pacemaker of FIG. 1 in accordance with the ACP,ARV, and other embodiments of the invention, is shown. The method shownin FIG. 4 shows the ACP embodiment described above. The method shown inFIG. 5 is the first version of the ARV embodiment described above. Themethod shown in FIG. 6 is the second version of the ARV embodimentdescribed above. These embodiments are aimed at alternative methods ofpreventing atrial competition, which atrial competition may cause atrialarrhythmias. In contrast, the method shown in FIG. 7 is aimed ataccurately detecting an atrial arrhythmia, and responding to sucharrhythmia, once detected. It is noted that the method shown in FIG. 7may be practiced alone or in combination with the methods shown in FIGS.4, 5 or 6.

In each flowchart, it is noted that the various steps of the particularmethod being depicted are shown in abbreviated form in "blocks" or"boxes," each of which has a reference numeral associated therewith.

Referring to the flowchart shown in FIG. 4, for example, a method ofoperating a rate-responsive, dual-chamber pacemaker is shown that avoidsgenerating atrial stimulation pulses that might compete with naturalatrial activity. It is understood that the pacemaker is programmed tooperate in a DDDR mode of operation, and in fact such is shown as afirst step (block 160) in FIG. 4. Basically, the method includes thesteps of: (a) sensing intrinsic P-waves; (b) determining if a givensensed intrinsic P-wave occurred during a post ventricular atrialrefractory period (PVARP) associated with the programmed DDDR mode ofoperation of the pacemaker, such PVARP being generated by the pacemakersubsequent to any sensed or paced ventricular activity; (c) in the eventthat a sensed intrinsic P-wave does occur during PVARP, as determined instep (b), starting an atrial competition prevention (ACP) period; and(d) inhibiting any atrial stimulation pulse in the event the ACP period,if any, has not timed out by the time the atrial stimulation pulse wouldotherwise be generated by the programmed DDDR mode of operation, oralternatively, delaying the generation of the atrial stimulation pulseuntil the ACP period has timed out.

More particularly, and as shown in FIG. 4, assuming that the DDDR pacingmode has been initiated (block 160), the method includes as a next stepsensing any P-waves (block 162). The term "P-wave" as used in FIG. 4(and the other flowcharts) is intended to mean "atrial activity". If aP-wave is sensed, then a determination is made as to whether the PVARPhas timed out (block 164). If so, then the pacemaker operates inaccordance with the conventional DDDR pacing mode (block 180), i.e., anA-pulse is generated at the conclusion of the SIR VAD only if anotherP-wave is not sensed before the timing out of the SIR VAD, with a V-Adelay being initiated by any atrial activity, either the sensing of aP-wave, or the generation of an A-pulse; and with a V-pulse beinggenerated at the conclusion of the A-V delay only if an R-wave is notsensed before the timing out of the A-V delay.

If the PVARP has not timed out (block 164) upon the sensing of a P-wave(block 162), then an ACP interval is initiated. As indicated above, theACP interval is a fixed interval having a preferred duration of around300 milliseconds, although for some patients it may be appropriate tohave a duration of between 250 and 350 milliseconds, or other values.The duration of the ACP interval may be fixed by the manufacturer of thepacemaker, or programmably set by the physician at implant (and adjustedby the physician, as necessary, thereafter). Once the ACP interval isstarted, the sensed P-wave (sensed at block 162) is ignored (block 168),i.e., its occurrence has no further effect, and a determination is madeas to whether the SIR VAD has timed out (block 170). If not, then thesensing circuits remain active to sense if a P-wave does occur (block162).

If a P-wave is not sensed (block 162), a determination is made as towhether the ACP interval has been started (block 163). Suchdetermination only has applicability to a "second pass" through theP-wave sensing determination block (block 162). A "second pass" occurswhen a P-wave was sensed (block 162) during a "first pass," the PVARPhad not timed out (block 164), the ACP interval was started (block 166),and the SIR VAD had not timed out (block 170), thereby returning controlfor the "second pass" through the P-wave sensing determination block(block 162). If, after such second pass, the ACP has been started (block163), then a determination is made as to whether the SIR VAD has timedout (block 170).

Once the SIR VAD times out, another determination is made as to whetherthe ACP interval, if any, has timed out (block 172). If not, adetermination is made as to whether the inhibit option has been selected(block 173). The inhibit option is the option described above inconnection with FIG. 3(D) where the A-pulse is inhibited. If the inhibitoption is selected, then the A-pulse is inhibited (block 176). If theinhibit option has not been selected (block 173), then the delay option,described above in connection with FIG. 3(E), is presumed to beselected. As such, nothing happens until the ACP interval times out, asdetermined at block 172.

Upon the timing out of the ACP interval (block 172), an A-pulse isgenerated (block 174). In either event, i.e., after the generation of anA-pulse (block 174) or the inhibiting of an A-pulse (block 176), the A-Vdelay is begun thereafter (block 178), and the pacer continues tooperate in the normal DDDR mode (block 180). If DDDR pacing is tocontinue (block 182), which is a programmable selection, then theprocess repeats, with the sensing circuits eventually being placed in astate that allows them to sense a P-wave (block 162) at the appropriatetime during the DDDR pacing cycle.

Note from FIG. 4, that so long as a P-wave is sensed, and the PVARP hasnot timed out, the ACP interval will be started. If an ACP interval haspreviously been started, then the ACP interval is restarted. In thismanner, the latest atrial activity sensed during the PVARP triggers(restarts) the ACP interval. Hence, no A-pulse can be generated until atleast the timing out of the ACP interval after the latest atrialactivity that occurs during the PVARP.

Note also from FIG. 4 that if a P-wave is not sensed (block 162), andthe ACP has not been started (block 163), then a determination is madeas to whether the SIR VAD has timed out (block 165). If it has not timedout, then the system continues to look for the occurrence of a P-wave(block 162). If it has timed out, then an A-pulse is generated (block174), in conventional demand pacer operation.

Referring next to FIG. 5, an alternative method is illustrated aimed atreducing the likelihood of atrial competition in a rate-responsive,dual-chamber pacemaker programmed to operate in a DDDR mode ofoperation. Essentially, this method includes the steps of: (a) sensingintrinsic P-waves; (b) determining if the sensed intrinsic P-waves areoccurring at an intrinsic atrial rate that is increasing and is at leastas rapid as a reference rate; (c) in the event that a sensed intrinsicatrial rate is increasing and is at least as rapid as the referencerate, as determined in step (b), shortening a post ventricular atrialrefractory period (PVARP) associated with the DDDR mode of operation;and (d) operating the pacemaker in the DDDR mode of operation with theshortened PVARP for so long as the sensed intrinsic atrial rate is equalto or greater than the reference rate.

More particularly, as seen in FIG. 5, after beginning the DDDR pacingmode (block 190), the intrinsic atrial rate, A_(R), is monitored (block192). Any conventional means may be used to determine A_(R). Forexample, the P-P interval may be measured and averaged over the previousn cardiac cycles, where n is an integer, such as 5. Once A_(R) has beendetermined, a determination is made whether A_(R) is approaching areference rate, REF_(R) (block 194). (It is to be understood thatalthough FIG. 5 speaks in terms of monitoring and measuring "rates," thesame result is obtained by monitoring and measuring "periods," as one isthe simply the reciprocal of the other.) Next, a determination is madeas to whether A_(R) is approaching REF_(R) (block 194). That is, adetermination is made as to whether the rate of the intrinsic P-waves isincreasing sufficiently fast so that a given P-wave might fall withinthe PVARP.

The determination as to whether a given P-wave might fall within thePVARP involves making an estimate based on the trend that has beenobserved in the increasing intrinsic P-wave rate. This estimate is madeusing any suitable determining technique. For example, the value of themost recent P-P interval (or the most recent P-P interval average),PP_(i) is compared with a second most recent P-P interval (or the secondmost recent P-P interval average), PP_(i-1), in order to determine howmuch the P-P interval has changed, ΔP-P. This ΔP-P value is thensubtracted from PP_(i) in order to get an estimate of what the next P-Pinterval, PP_(i+1), is likely to be if this same trend continues. (Notethat for an increasing intrinsic P-wave rate, the P-P intervaldecreases.) This estimate of PP_(i+1) is compared with a correspondinginterval, PP_(R), associated with the REF_(R). The value of PP_(R) isselected to be just slightly longer than the ARP. Hence, if PP_(i+1) isless than PP_(R), it means that the next P-wave will likely fall withinPVARP.

Once the determination has been made (at block 194) that A_(R) isapproaching REF_(R), and hence that a P-wave may soon fall in the PVARP,is automatically shortened (block 198), providing that PVARP has notpreviously been shortened (block 196). The amount by which PVARP isshortened is programmably selected by the physician, and will typicallybe 50 to 200 milliseconds. Once PVARP is shortened, the pacer operatesin the DDDR pacing mode using the shortened PVARP.

Whether operating the pacer using the shortened PVARP, or using thenormal PVARP, the determination (made at block 194) as to whether A_(R)is approaching REF_(R) can also be used as an indication as to whetherA_(R) is not approaching REF_(R), i.e., whether A_(R) is sufficientlyremoved from REF_(R) to assure that the P-waves will not fall within thePVARP. If the P-waves are not likely to fall within the original PVARP,and assuming that PVARP has been previously shortened (block 200), thenthe value of PVARP is returned to its original value (block 202).Thereafter, the pacer operates in its DDDR mode (block 204), using thenormal PVARP or the shortened PVARP, depending on whether the intrinsicP-waves are likely to fall within the normal PVARP or not, untilprogrammed otherwise (block 206).

Referring next to FIG. 6, a variation of the method shown in FIG. 5 isillustrated. The method illustrated in FIG. 6 is essentially the same asmethod shown in FIG. 5 except that rather than shortening PVARP aprescribed amount upon making a determination that A_(R) is approachingthe reference value REF_(R), PVARP is adjusted as an inverse function ofthe sensor-indicated rate, SIR. Thus, upon initiating DDDR pacing usingthis variable PVARP approach (block 191), the intrinsic atrial rateA_(R) is monitored (block 193). If a determination is made that A_(R) isgreater than or equal to a reference rate, REF_(R) (block 195), whichreference rate may be quite low, e.g., as low as the at rest rate of thepacemaker, the PVARP is adjusted as a function of the SIR (block 205),and the pacer continues to operate in the DDDR pacing mode (block 201).The relationship between PVARP and the SIR is preferably programmable,and causes the PVARP to shorten a first prescribe amount for eachmeasured increment in the SIR. Thus, for example, an increase in the SIRof 10 ppm causes the PVARP to shorten 10 or 20 milliseconds. Similarly,a decrease in the SIR of 10 ppm causes the PVARP to lengthen 10 or 20milliseconds.

Should A_(R) fall below the reference rate REF_(R) (block 195), thenPVARP is restored to its original value (block 199) providing PVARP hasnot been previously adjusted (block 197). Thus, by selecting REF_(R) tobe an appropriate value, any increase of A_(R) above REF_(R) causesPVARP to gradually shorten and then gradually lengthen until A_(R) isback below REF_(R), at which time PVARP remains at its original value.However, any initial decreases of A_(R) below REF_(R) do not cause thevalue of PVARP to be change. Thus, PVARP changes as controlled by theSIR only when A_(R) initially increases, not when it initiallydecreases.

Referring next to FIG. 7, a still further embodiment of the invention isshown in flowchart form. This embodiment of the invention, like theembodiments described above, provides a method of operating arate-responsive, dual-chamber pacemaker programmed to operate in a DDDRmode of operation (block 210). In contrast to the methods describedabove in connection with FIGS. 4, 5 and 6 (which are aimed at preventingatrial competition), the method shown in FIG. 7 is aimed at detecting anatrial arrhythmia and dissociating the ventricular paced rate from itonce detected. As previously indicated, such method may be practicedalone or in combination with the methods of FIGS. 4, 5 or 6.

It is understood that the pacemaker with which the method of FIG. 7 isused includes physiological sensor means for defining a sensor-indicatedrate (SIR) indicative of a preferred rate at which the pacemaker shouldpace a patient's heart on demand based on a sensed physiologicalparameter. Broadly stated, the method comprises the steps of: (a)sensing intrinsic P-waves; (b) determining if the sensed intrinsicP-waves are occurring at a rate that is much greater than the SIR; (c)in the event that a sensed intrinsic atrial rate is greater than theSIR, as determined in step (b), lengthening a maximum tracking interval(MTI) associated with the DDDR mode of operation of the pacemaker,thereby reducing a maximum tracking rate associated with the DDDR modeof operation; and (d) operating the pacemaker in the DDDR mode ofoperation with the lengthened MTI for so long as the sensed intrinsicatrial rate is equal to or greater than the SIR.

More particularly, and with reference to FIG. 7, it is seen that themethod begins with a first step of pacing in a DDDR mode (block 210).Thereafter, the intrinsic atrial rate A_(R) is monitored (block 212).Even when P-waves occur during PAARP, such P-waves are still monitoredso that A_(R) can be determined. The value of the intrinsic atrial rate,A_(R), is compared to the sensor-indicated rate, SIR. If A_(R) issignificantly greater than SIR (block 214), the maximum tracking rate(MTR) of the pacemaker is reduced (block 218) providing it has notpreviously been reduced (block 216). If A_(R) is not significantlygreater than the SIR (block 214), and if the MTR has not been previouslyreduced (block 222), the pacemaker continues to operate in its DDDR mode(block 220). If the MTR has been previously reduced, but a determinationis made (at block 214) that A_(R) is not significantly greater than theSIR, then the MTR is restored to its original value (block 224), and thepacer thereafter operates in accordance with its DDDR mode (block 220)for so long as the DDDR mode remains as the programmed mode (block 226).

Note, that the maximum tracking rate, or MTR, of a pacemaker isdetermined by a programmably set maximum tracking interval (MTI), anddefines the highest rate at which the pacemaker can track intrinsiccardiac activity in order to provide stimulation pulses on demand. (Amore complete description of the MTR, and associated MTI, may be foundin the Mann et al. '980 patent, and/or the Thornander et al. '555patent, previously referenced.)

It is noted that a determination that A_(R) is significantly greaterthan the SIR (block 214) provides a reliable indication (assuming thatthe physiological sensor used with the pacemaker is functioning) that anatrial arrhythmia condition is present. That is, if the fast atrial ratewere due to increased physiological demand, then the SIR would increasecommensurate with the atrial rate. However, when there is a largedisparity between A_(R) and the SIR, then that is an indication thatsomething is wrong. Advantageously, reducing the maximum tracking rateof the pacer will minimize patient symptoms related to the fast atrialrate. Additionally, should the fast detected atrial rate be related to1:1 retrograde VA conduction, the reduction in the paced ventricularrate will be beneficial. If the arrythmia is stops, then A_(R) shouldreturn to a rate that is close to the SIR, and the MTR can be returnedto its original value (block 224). (Note that reducing the MTR istypically realized by increasing the duration of length of the MTI.) Thevalue of A_(R) required to be above the SIR to produce this response (asdetermined at block 214) may be preset by the manufacturer or a valueprogrammable by the physician.

Still referring to FIG. 7, if A_(R) is significantly greater than theSIR (block 214), and if the MTR has already been reduced (block 216),then a further determination is made as to whether A_(R) has beensignificantly greater than SIR for a sustained period of time (block228). A sustained period of time is defined as either a fixed period oftime, such as 60 seconds, or a prescribed number of cardiac cycles, m,where m is an integer, such as 150. If a sustained period of time hasnot elapsed since the MTR was first reduced (as determined at block228), then the pacer continues to operate in its programmed DDDR mode(block 220). If, however, a sustained period of time has elapsed sincethe MTR was first reduced, i.e., if the atrial arrhythmia has persistedfor the defined sustained time period, then the pacing mode isautomatically switched to a single-chamber pacing mode (block 230), suchas the VVIR mode. Operation begins in the new mode (block 232) until areprogramming change is made (block 234), or until the detected atrialrate A_(R) reduces to an acceptable level (blocks 236-242).

It is noted that if the arrhythmia continues for a sustained period oftime (block 228), that is an indication that something is wrong, andswitching to an alternate mode (block 230), such as the VVIR mode,advantageously provides the safest and most effective way of dealingwith the problem until such time as a physician can thoroughly evaluateexactly what the problem is and what can be done to correct it.

In conclusion, and as described above, it is seen that the presentinvention provides a dual-chamber pacemaker operating in a sensor-drivenmode, and a method of operating such a pacemaker, that prevents atrialarrhythmias. This it does by minimizing the likelihood of atrialcompetition between naturally occurring P-waves and atrial stimulationpulses. In one embodiment, termed the "ACP" embodiment, the pacemakersenses atrial activity during the pacemaker's post ventricular atrialrefractory period (PVARP) and prevents any atrial stimulation pulsesfrom being generated within a prescribed time interval thereafter,thereby minimizing atrial competition by assuring that there is alwaysat least the prescribed time interval between sensed atrial activity andan atrial stimulation pulse. In another embodiment, termed the ARVembodiment, the pacemaker automatically shortens PVARP whenever thesensor-driven rate approaches a rate that might place atrial stimulationpulses near the end of the time in the cardiac cycle when the original(unshortened) PVARP terminates, thereby minimizing atrial competition byincreasing the time period (after PVARP) during which atrial activitycan be sensed, which atrial activity (if sensed after PVARP) inhibitsthe generation of an atrial stimulation pulse.

As further seen from the preceding description, the present inventionprovides a dual-chamber, sensor-driven pacemaker and method of operationthat ensures that when an atrial stimulation pulse is provided, itcaptures the heart. This it does by ensuring that the atrial stimulationpulse is not provided at a time that would compete with a naturallyoccurring P-wave, during which time the cardiac tissue is refractory andunable to respond properly to the stimulation pulse.

As also seen from the above description, the present invention providesa dual-chamber, programmable, rate-responsive pacemaker that includesselectable means for detecting and responding to an atrial arrhythmia.This is accomplished by programming the pacemaker to detect an atrialarrhythmia whenever there is a large disparity between a sensed atrialrate and a sensor-driven rate. Once such atrial arrhythmia is detected,the pacemaker includes means for automatically reducing the maximumtracking rate of the pacemaker, and/or switching the pacemaker operatingmode from a dual-chamber rate-responsive mode, such as DDDR, to asingle-chamber ventricular rate-responsive mode, such as VVIR.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. In a rate-responsive, dual-chamber pacemakerconfigured to operate in a DDDR mode of operation, a system forpreventing atrial competition comprising:means for defining aphysiological pacing rate; control means for generating timing signalsindicative of when an atrial and/or ventricular stimulation pulse shouldbe generated by said pacemaker in order to maintain said physiologicalpacing rate; sensing means coupled to said control means for sensingatrial and ventricular activity, such as P-waves, indicating naturalatrial activity, and R-waves, indicating natural ventricular activity,said control means generating the timing signals needed to generateatrial and/or ventricular stimulation pulses on demand as needed in theabsence of intrinsic P-waves and/or R-waves; stimulation pulsegenerating means coupled to said control means for generating saidatrial and/or ventricular stimulation pulses in response to said timingsignals; said control means including: PVARP generating means forgenerating a post ventricular atrial refractory period (PVARP)subsequent to the generation of each ventricular stimulation pulse orthe sensing of an R-wave, said PVARP defining a time interval duringwhich sensed atrial activity is not considered as a valid P-wave, saidPVARP generating means generating at least a first PVARP and a secondPVARP; and atrial pulse prevention means for preventing an atrialstimulation pulse from being generated that is in competition withatrial activity sensed during said PVARP.
 2. The rate-responsivedual-chamber pacemaker, as set forth in claim 1, wherein said atrialpulse prevention means comprises:rate determining means for determiningan intrinsic atrial rate; first comparison means for determining if saidsensed intrinsic atrial rate is approaching a reference rate; and meansresponsive to said first comparison means for changing said first PVARPto a second PVARP, said second PVARP being different than said firstPVARP.
 3. The rate-responsive dual-chamber pacemaker, as set forth inclaim 2, wherein said second PVARP is approximately 50 to 200milliseconds shorter than said first PVARP.
 4. The rate-responsivedual-chamber pacemaker, as set forth in claim 2, wherein said secondPVARP changes as an inverse function of said physiological pacing rate.5. The rate-responsive dual-chamber pacemaker, as set forth in claim 4,wherein said second PVARP changes as an inverse function of saidphysiological pacing rate only if said physiological pacing rate firstincreases.
 6. A method of operating a rate-responsive, dual-chamberpacemaker so as to avoid generating atrial stimulation pulses that mightcompete with natural atrial activity, said pacemaker being programmed tooperate in a DDDR mode of operation so as to provide stimulation pulseson demand at a rate determined by a physiological sensor, said methodcomprising the steps of:(a) sensing intrinsic P-waves; (b) determiningif the sensed intrinsic P-waves are occurring at an intrinsic atrialrate that is at least as rapid as a reference rate; (c) in the eventthat a sensed intrinsic atrial rate is at least as rapid as saidreference rate, as determined in step (b), changing a post ventricularatrial refractory period (PVARP) associated with said DDDR mode ofoperation, and (d) operating said pacemaker in said DDDR mode ofoperation with said changed PVARP for so long as the sensed intrinsicatrial rate is equal to or greater than said reference rate.
 7. Themethod, as set forth in claim 6, further including the step of returningPVARP to its original value in the event that the sensed intrinsicatrial rate is not increasing and is less than said reference rate. 8.The method, as set forth in claim 7, wherein the step of changing saidPVARP comprises shortening said PVARP.
 9. The method, as set forth inclaim 8, wherein the step of shortening said PVARP comprises shorteningit a prescribed amount.
 10. The method, as set forth in claim 8, whereinthe step of shortening said PVARP comprises shortening it an amount thatis inversely proportional to the rate determined by said physiologicalsensor.